The trimming (adjustment) of electrical resistors is a widely used procedure in the manufacture of microelectronics and electronic components, and in common design of user circuits, especially where precision calibration is desired. In principle, one trims the resistor until an observable local or global circuit parameter reaches a desired value. Resistor trimming is widespread in both manufacturing of a variety of components and instruments, and in the user community.
It is known that certain thermally-mutable materials, such as polycrystalline silicon (polysilicon) or polycrystalline SiGe, respond to heating or voltage pulses above a certain threshold, by changing their electrical resistivity. Several publications describe experiences whereby the resistance is deliberately “trimmed” to a target value within a certain level of precision.
Amemiya et al (Y. Amemiya, T. Ono, K. Kato Electrical Trimming of Heavily Doped Polycrystalline Silicon Resistors IEEE Trans. Electron. Dev. vol. ED-26 (1979), 11, pp. 738-742; and US patent #4210996-1980), describe using applied voltage pulses directly to polysilicon resistors, where these trimming pulses have duration in the range of 0.5 us to 1 ms. The effect of these pulses is to trim “down” (reduce the resistance). They state that shorter pulses require higher voltages (24-28V) than longer pulses, to achieve the trimming effect. For example, 0.5 us pulses yielded 0.02% resistance decrease. They also state that the resistance can be increased and so called “recovery” can be accomplished, even to resistance values higher than the initial (as-fabricated) resistance value, Rinit. They state that the “Time constant for recovery is larger than that of the trimming process.” They disclose 0.01% (100 ppm) trimming accuracy.
Kato et al (K. Kato, T. Ono, Y. Amemiya A Monolithic 14 Bit D/A Converter Fabricated with a New Trimming Technique (DOT), IEEE J. Solid-State Circuits vol. SC-19 (1984), 5, pp. 802-807), also apply voltage pulses directly to polysilicon resistors, and describe a technique for stabilizing the post-trimming resistance, called the Excess Trimming and Restoration (ETR) technique. They describe that the “Trimmed R shows a slight recovery when it's left at high T for a long time” and state that it is necessary to suppress this recovery. They describe a technique by which the “resistor is intentionally over-trimmed and then restored to its required value”, indicating that this recovery requires lower current. They also report 0.01% (100 ppm) accuracy, using this technique.
Feldbaumer et al (D. Feldbaumer, J. Babcock, V. Mercier, C. Chun, Pulse Trimming of Polysilicon Resistors IEEE Trans. Electron. Dev. vol. ED-42 (1995), 4, pp. 689-695), also apply voltage pulses directly to polysilicon resistors. To trim down, they apply a series of heat pulses of increasing amplitude, having 3 ms duration, with 10 ms between each pulse. They accomplished recovery, by “reapplying a current of less than the final trim current but above threshold” (where the “threshold” is the threshold below which the resistance doesn't change.) They report that the “Magnitude of recovery is small in comparison to the amount which is trimmed” for example less than 1% after trimming “down” by 20% of the as-fabricated value. They suggest that this “Offers a method to finely tune resistors which are trimmed past their target” and suggest that there is an optimal recovery current, in the range of 80% to 90% below the current of the last previous “down” pulse. They use only constant-amplitude pulses with total accumulated time of approximately 25 ms. For example they use a “Trim Pulse Width” of 1 ms and “Recovery Pulse Width” of 2.5 ms with Total Accumulated Recovery Time of 25 ms (FIG. 5 of their paper), and state that they could recover by about 1% to 2% of the resistance value using total accumulated pulse times on the order of a second or less.
Babcock et al (J. Babcock, P. Francis, R. Bashir, A. Kabir, D. Shroder, M. Lee, T. Dhayagude, W. Yindeepol, S. Prasad, A. Kalnitskiy, M. Thomas, H. Haggag, K. Egan, A. Bergemont, P. Jansen, Precision Electrical Trimming of very Low TCR Poly-SiGe Resistots IEEE Electron. Dev. Letters, vol. 21 (2000), 6, pp. 283-285), also use auxiliary heaters to heat integrated Si—Ge resistors. They disclose that they are able to recover up to ˜20% higher than the as-fabricated resistance, (Rinit), by applying a constant-amplitude current for 40000 s. In this publication, they also demonstrate that the TCR of a trimmed resistor varies with (is a function of) the trimmed resistance.
Babcock et al have also described in provisional patent application US 2002/0035519 a procedure of trimming involving applying a sequence of pulses having increasing amplitude (each pulse greater than the previous).
Therefore, the prior art shows methods of both increasing and decreasing the resistance of certain thermally-mutable materials, but there remain significant limitations of trimming performance. Most importantly, increasing the resistance by these methods appears to take very long times, or is quite limited in range (e.g. 1%-2%). It would be extremely advantageous, for the purpose of circuit design, to increase the precision to which a resistor can be trimmed and have this precision be achievable over a short period of time.